The first generation Fiesta came out in 1976. Top of the line model had the Valencia Kent series 771 motor ( 77 for 1977 ). The block is shorter than the Kent block used in RWD cars but a lot of parts are interchangeable between the two. It's not a good idea to use a Valencia block in a RWD car.

I too have a 771m in my engine collection, the block is the same length but the engine is shorter because of the shorter timing chain cover and rear cover. The crank too is a little shorter but interchangeable, I think the outer main bearing shells are narrower and there is a hole that needs to be drilled and tapped at the water pump. The block is every bit as good as the 711M block strength wise and is a good swap candidate. If you had a 771M block you could easily just drop in any other x/flow crank with it's bearings and use the front and rear covers and oil pan also from the earlier x/flow. If I was at home right now I could go and measure it but I do have some pictures here with me.
I have heard that some of the Fiesta blocks did not have the side engine mount lugs cast into the block??? They are all present on mine, the water and oil pumps are different too but the bolt patterns the same except for the one water pump hole I mentioned earlier. These differences are likely why they say it's not recommended to use for a swap but if you have a second donor x/flow you can have an engine equal to a 711M.
Here is a comparison.

I think the 711 block has the heavy duty square main caps and wider diameter stems / lifter faces.
It is possible to use the 771 block instead of the English Kent block for a RWD. You would need to remove 8mm from each mounting web on the left side of the block then drill further in and tap. Also you would need to either drill or increase the hole size for the water pump / generator bracket.
I had to drill a hole in the oil pump for an oil pressure indicator when i used a 771 block in a Anglia 105E.

Some of these above things you have already mentioned and like you said you would need a donor Kent block.

You are right, the 711M does have the big, square main caps, wide lobe cam and larger cam followers. The 771M also has the big beefy main caps and wider cam lobe and followers. You are also correct about the left engine mounting web, the 771M is 8mm more than the early x/flows BUT the same as a 711M so if you were putting a 711M into a place where the EARLY x/flow came out you would have to trim the mount again just as a 771M. If you were replacing a 711M then no trimming will be needed as they are the same. Here is a bit of trivia (true or false?) The 711M engine is wider to be the same fit as the German made 2000 SOHC engine. Weather that is true or not I did discover when putting a Pinto 2000cc engine into a 105e that the engine just bolted directly to the 105e mounts and Cross member with no problem EXCEPT I had to trim 8mm of the block casting on the left side.
This sort of supports the Idea that it may have been designed that way, when the Ford Pinto first came out it was a 711M 1600 before becoming the German made 2000 SOHC engine. They both take the same engine mounts and are interchangeable._________________Anglia 100e modified
Prefect 100e stock

i can confirm about the 711 specs as above. also, the fwd and rwd x flows are different when it comes to mounting them and it's just never worth the hastle when there are more or atleast just as many rwd suited crossers out there.